Add stage-3 content triage: sort unknown files into binary/ and text/

Unknown files are no longer terminal. Stage 1 now routes :unknown onto a
dedicated queue (with the same blocking backpressure as the known queue),
and a third worker pool sorts each file into data/binary/ or data/text/
using a NUL-byte sniff of the first 8000 bytes.

- content.jl: is_binary content sniff (stage 3)
- worker.jl: handle_unknown_job; stage-1 routes unknown with backpressure;
  KNOWN_ENQUEUE_RETRY_SECONDS -> ROUTE_ENQUEUE_RETRY_SECONDS (serves both)
- config.jl: unknown_worker_count/queue_capacity, binary_dir, text_dir + env
- FileServer.jl: unknown queue, pool, stage-aware recovery, drain ordering
- tests for is_binary and handle_unknown_job; tmp_config isolates new dirs
- README: three-stage pipeline

test/runtests.jl

@@ -7,7 +7,8 @@ using JSON3
 # directly rather than only through the HTTP surface.
 using FileServer: Job, Config, ChannelQueue, enqueue!, dequeue!, length,
                   sanitize_filename, recover_dir!, normalize_metadata,
-                  build_metadata, finalize_known!, run_exiftool
+                  build_metadata, finalize_known!, run_exiftool,
+                  is_binary, handle_unknown_job
 
 # A minimal, valid 1×1 PNG. Lets the real-exiftool tests assert stable facts
 # (FileType == "PNG", 1×1 dimensions) that don't drift across exiftool versions.
@@ -21,6 +22,8 @@ function tmp_config(root; kwargs...)
         spool_dir   = joinpath(root, "spool"),
         known_dir   = joinpath(root, "known"),
         unknown_dir = joinpath(root, "unknown"),
+        binary_dir  = joinpath(root, "binary"),
+        text_dir    = joinpath(root, "text"),
         done_dir    = joinpath(root, "done"),
         failed_dir  = joinpath(root, "failed"),
         kwargs...,
@@ -138,6 +141,52 @@ end
         end
     end
 
+    @testset "is_binary: NUL-byte sniff" begin
+        mktempdir() do root
+            # Plain text → text.
+            txt = joinpath(root, "notes.txt")
+            write(txt, "hello, world\nsecond line\n")
+            @test is_binary(txt) == false
+
+            # A NUL byte anywhere in the sniff window → binary.
+            bin = joinpath(root, "blob.dat")
+            write(bin, UInt8[0x01, 0x02, 0x00, 0x03])
+            @test is_binary(bin) == true
+
+            # Empty file has no NUL → treated as text.
+            empty = joinpath(root, "empty")
+            write(empty, UInt8[])
+            @test is_binary(empty) == false
+
+            # A NUL past the sniff window is not seen → still text.
+            far = joinpath(root, "far.txt")
+            write(far, vcat(fill(UInt8('a'), FileServer.CONTENT_SNIFF_BYTES), UInt8[0x00]))
+            @test is_binary(far) == false
+        end
+    end
+
+    @testset "handle_unknown_job: routes to binary/ and text/" begin
+        mktempdir() do root
+            cfg = tmp_config(root)
+
+            # A binary file (embedded NUL) lands in binary/.
+            bpath = joinpath(cfg.unknown_dir, "id-b-blob.dat")
+            write(bpath, UInt8[0x00, 0xFF, 0x10])
+            bjob = Job("id-b", "blob.dat", bpath, filesize(bpath), 0.0)
+            handle_unknown_job(bjob, cfg, 1)
+            @test isfile(joinpath(cfg.binary_dir, "id-b-blob.dat"))
+            @test !isfile(bpath)
+
+            # A text file lands in text/.
+            tpath = joinpath(cfg.unknown_dir, "id-t-notes.log")
+            write(tpath, "just some log text\n")
+            tjob = Job("id-t", "notes.log", tpath, filesize(tpath), 0.0)
+            handle_unknown_job(tjob, cfg, 1)
+            @test isfile(joinpath(cfg.text_dir, "id-t-notes.log"))
+            @test !isfile(tpath)
+        end
+    end
+
     @testset "recover_dir!: re-enqueues work, skips sidecars" begin
         mktempdir() do root
             dir = joinpath(root, "known"); mkpath(dir)